when changing plugs yesterday, I found one that the socket would not fit on.
Upon further investigation I noticed that it was rounded off. What should I try to get this (1) plug out?(without breaking the plug and really being screwed)

Try a Snap-On socket, they will bite in when other will not. Or try using a metric deep socket, it may fit a little tighter and may need to be tapped on. I would spray some WD-40 or simular around base for plug. Just in case its siezed.